If I write a blog a post about how I got 50 billionaires to attend my hackathon then the techniques for attracting those billionaires might be considered useful, the fact I was restricting my hackathon to mainly billionaires doesn't negate the benefit of those techniques. So I can have two titles 'How I got 50 billionaires to attend my hackathon' or 'It's easy to get 50 billionaires to attend a hackathon just refuse anyone who isn't a billionaire'. The trick here isn't to limit attendees, that's easy, the trick is to encourage the attendees you would like to attend. If it's billionaires then you should definitely check out my upcoming series of blog posts on how I got 50 billionaires to attend my hackathon (possibly fictional), if it's women (who are massively underrepresented at hackathons, much like billionaires) then perhaps the linked article would be valuable.